If you experience an authorization error from Google's advanced protection program when signing up, there are a few additional steps you can take to still use Shortwave as trusted app.
If you are using a Google Workspace account and are an Admin, you can whitelist Shortwave as a trusted app. Here's how:
Go to your Google Admin Console
At the top of the Configured apps list, click Add app
Choose OAuth App Name or Client ID
Enter the app's name: "Shortwave" and then click SEARCH
From the list of search results, click Select for the app that you want to manage
Click the top check the box to select all OAuth Client IDs and then click SELECT
Choose: Trusted and then click CONFIGURE
After that, you should be able to sign into Shortwave. If you run into any trouble getting set up, please email firstname.lastname@example.org for assistance.
If you are using a personal Gmail account, unfortunately you have to un-enroll to use Shortwave since it is not an official Google app yet.